A rapidly-swept high-sensitivity cavity ringdown spectrometer (SC-CRDS) has been developed based on near-infrared continuous-wave diode lasers. By rapid sweeping/dithering of the ringdown cavity length via a piezo-electric transducer (PZT) , in combination with an automatic cavity length tracking circuit to follow laser wavelength changes, fast CRDS measurements are achieved. This system is applied for measurement of CH4 concentration by detecting its spectroscopic absorption peak around 1653.73 nm (6046.95 cm- 1). By measuring multiple spectral points around the absorption peak, both the peak absorption value and corresponding laser wavelength are determined. A feedback control to the laser wavelength helps to stabilize its center wavelength to the absorption peak. Therefore, it successfully solves the problem of strong wavelength drift of the laser in free operation. The line strength of the CH4 1653.73 nm absorption feature is calibrated by measuring CH4 premixture samples with reference concentrations. This simple CRDS system shows stable and reliable performance as well as a CH4 concentration detection limit of 1.0×10-9; and it can be applied to monitor CH4 in ambient air over a long time.